The present invention relates to a hydraulic autotensioner for maintaining a constant tension on a belt, such as a timing belt for driving cam shafts of an automotive engine.
A conventional hydraulic autotensioner of this type is disclosed in Unexamined Japanese Patent Publication 2-245555.
Referring to FIG. 5, the conventional autotensioner includes a cylindrical body 1, and a plunger 2. The plunger 2 is slidably mounted in the body 1 and has a recess 3 formed in the top thereof. A rod 5 slidably extends through a seal member 4 provided in a top opening of the body 1 and has a bottom end which is received in the recess 3. A pressure control spring 6 is mounted in the body 1 and biases the belt tension control member A, which includes the plunger 2 and the rod 5. The control member A is biased in a direction such that the control member protrudes from the body 1 so as to apply tension to a belt 9 through a tension pulley 8 carried by a pivotable pulley arm 7.
The plunger 2 of the belt tension control member A partitions the interior of the body 1 into a pressure chamber 10 and a reservoir chamber 11. The chambers 10 and 11 communicate with each other through a return passage 12. A check valve 13 is provided which checks the flow of hydraulic oil from the pressure chamber 10 into the reservoir chamber 11 through the return passage 12.
When the tension in the belt 9 increases, the rod 5 is pushed downward (in FIG. 5), so that the pressure in the pressure chamber 10 increases. Namely, the pressure in the pressure chamber 10 and the biasing force of the pressure control spring 6 resist the pressure applied to the rod 5 by the belt.
When the downward pressure applied to the rod exceeds the sum of the pressure in the pressure chamber 10 and the biasing force of the pressure control spring 6, the hydraulic oil in the pressure chamber 10 begins to leak into the reservoir chamber 11 through a narrow gap 14 defined between the inner surface of the body 1 and the plunger 2.
When the tension in the belt 9 begins to decrease, the pressure control spring 6 pushes the belt tension control member A out of the body 1, thus re-tensioning the belt 9.
When the plunger 2 begins to move upwardly due to decreasing tension in the belt, the check valve 13 instantly opens return passage 12, thus allowing hydraulic oil in the reservoir chamber 11 to flow back into the pressure chamber 10 through the return passage 12, so that the belt tension control member A can quickly move toward the belt. Any slack of the belt is thus absorbed by the extension of belt tension control member A.
One important requirement for such an autotensioner is that the belt tension control member A must be able to respond quickly to sudden slackening of the belt. If the response time is poor, the belt can skip a tooth or two of any gear with which it engages. In order for the belt tension control member A to smoothly protrude from the body if the belt slackens suddenly, the flow resistance encountered by the hydraulic oil while flowing through the return passage 12 has to be as small as possible.
In the conventional autotensioner, the return passage 12 formed in the belt tension control member A is formed by an axial hole 15 formed in the plunger 2, and a T-shaped hole 16 formed in the rod 5. Forming the T-shaped hole 16 is time-consuming and expensive because it is formed by drilling. Moreover, burrs tend to form at the junction between the axial portion 16a and the diametric portion 16b of the T-shaped hole 16. In order to remove such burrs, the hole 16 which is formed by drilling has to be reamed, which further increases the trouble and costs associated with forming the hole 16.
An object of the present invention is to provide an autotensioner which has a return passage through which the pressure chamber can communicate with the reservoir chamber and which can be formed easily at low cost.